The present invention relates to an ultraviolet-curable organopolysiloxane composition or, more particularly, to an ultraviolet-curable organopolysiloxane composition which is cured with a relatively small dose of ultraviolet irradiation at room temperature and is useful as a coating agent of release paper.
It is a well established technology that an ultraviolet-curable organopolysiloxane composition is applied to the surface of a substrate material and irradiated with ultraviolet light so that the coating film is cured to give a thin surface film of the cured organopolysiloxane composition having surface releasability. Several different types of ultraviolet-curable organopolysiloxane compositions suitable for such an application are known in the prior art including a composition comprising an organopolysiloxane having epoxy-functional groups and an onium salt-based compound as a photopolymerization initiator disclosed in Japanease Patent Kokai 56-38350 and a composition comprising an organopolysiloxane having acryloxy-functional groups and a free-radical photopolymerization initiator disclosed in Japanease Patent Publication 53-36515.
The cured surface film of each of these known compositions, however, has a surface property inherent in the epoxy-functional groups or acryloxy-functional groups and it is a difficult matter to obtain a cured surface film having the advantageous intermediate properties originating in both types. In this regard, an improved ultraviolet-curable organopolysiloxane composition is proposed in Japanese Patent Kokai 58-213024 according to which an organopolysiloxane having epoxy-functional and/or acryloxy-functional groups is admixed with a cationic onium salt-based photopolymerization initiator and a free-radical photopolymerization initiator in combination. This composition, however, is defective because full curing of the composition can be achieved so as to prevent migration of the silicone constituent from the surface after curing only by the combined use of two different types of photopolymerization initiators.